Quantitative modeling of Sr, Ca, Rb, and K in the Bjerkrem-Sogndal layered lopolith (S.W. Norway)

Abstract

A systematic approach with graphic techniques is used to establish a quantitative model of fractional crystallization process in igneous layered complexes. Modeling of the evolution of Sr-Ca in plagioclase and K-Rb in plagioclase and whole rock coming from the Bjerkrem-Sogndal layered lopolith (Rogaland-S.W. Norway) is taken as an example. The relationships in logarithmic coordinates can be decomposed in a succession of segments. This permits identifying the Rayleigh law as controlling the process. A step by step solution is used to determine the parameters of the model which lead to the adjustment of the calculated evolution to the observed trend. Evidence in favour of an open system crystallization of the cumulate rocks permits determining the equilibrium partition coefficients between the various minerals and the liquid. The mean cumulate corresponding to a phase of crystallization of the intrusion is determined by averaging the mineral composition of the rocks belonging to that phase. The concentrations of the major elements Ca and K are used instead of activities. The adopted plagioclase-magma partition coefficients are close to those measured in anorthositic rocks for the same range of plagioclase composition between megacrysts and liquid. For an anorthite content of respectively 50, 43 and 31, DSrplagis equal to 2.0, 2.3 and 3.9, DKplagvaries between 0.40 and 0.25, DRbplagis either constant (ca. 0.10) or increases from 0.12 to 0.25, DCaplagis supposed to remain at an approximately constant value of 1.48. The fraction of residual liquid in the intrusion is 0.47 at the end of the anorthositic-leuconoritic phase, and 0.21 at the end of the cpxnoritic phase.